1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to equipment for transmission of encrypted data using quantum cryptography.
2. Description of the Related Art
Cryptography is often used to exchange messages between two or more nodes (users, stations) in enhanced or even perfect privacy. A typical cryptographic method employs a publicly announced encrypting/decrypting algorithm, with the confidentiality of transmitted information provided by a secret key used in conjunction with that algorithm. Usually, a secret key is a randomly chosen, sufficiently long sequence of bits known only to the transmitting and receiving parties. For example, in a symmetric ciphering scheme, the transmitting station encrypts information using the secret key and sends the encrypted data over a public channel to the receiving station. The receiving station then uses the same key to undo the encryption and recover the original information.
It is well known that the longer the key, the more secure the system. For example, one widely used encryption system, the Data Encryption Standard (DES), has a key length of 56 bits. No method substantially more efficient than trying all 256 possible values of the key is known for breaking the DES. However, it is still possible that, if an eavesdropper has substantial computational power, the DES can be defeated. Therefore, to achieve higher security, a one-time pad (i.e., a key that is as long as the transmitted message) can be used. Although a communication system employing one-time pads is theoretically secure against attacks based on sheer computational power, nevertheless, such a system has to deal with what is known as the key-distribution problem, i.e., the problem of securely furnishing keys to the transmitting/receiving stations.
With conventional (classical) key transmission methods, which can be subject to passive monitoring by an eavesdropper, it is relatively difficult to transmit a certifiably secret key, and cumbersome physical security measures are usually required. However, secure key distribution is possible with quantum techniques. More specifically, in quantum cryptography, a secret key is transmitted through a special quantum channel whose security is based on the principles of quantum mechanics. More specifically, it is known that any measurement of a suitably chosen quantum system inevitably modifies the quantum state of that system. Therefore, when an eavesdropper attempts to get information out of the quantum channel by performing a measurement, the fact that the measurement has been performed can be detected by legitimate users, who will then discard all compromised keys.
In practice, a quantum channel can be established using, e.g., (i) a train of single photons propagating through an optical fiber, with key bits encoded by the photon's polarization or phase, or (ii) a train of coherent optical pulses, each containing a small number (e.g., less than a few hundred) of photons, with key bits encoded by quadrature values of selected variables characterizing each pulse. More details on the establishment and use of representative quantum channels can be found, e.g., in a review article by N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, entitled “Quantum Cryptography,” published in Reviews of Modern Physics, 2002, vol. 74, pp. 145-195, the teachings of which are incorporated herein by reference.
Although some progress has been made in developing equipment for quantum channels, this equipment is still not up to the performance targets, e.g., in quantum-key distribution (QKD) rate and transmission distance. For example, a current commercially available QKD system offers a QKD rate of about 1.5 kb/s over a single-mode optical fiber having a length of about 25 km. For comparison, a representative classical communication system offers a data transmission rate of about 10 Gb/s over an optical fiber having a length of about 1000 km. Given these parameters for the QKD and classical systems, one finds that significant improvements in QKD rate and/or transmission distance are desirable.